1. Field of the Invention
This invention deals with accelerometers, more particularly with digital accelerometers.
2. Description of the Prior Art
Current accelerometers in use are typically of one of two types. The first type uses strain-sensing techniques. These have piezo-electric or piezo-resistive material that senses either the stress induced by the inertial mass, or the strain induced by the spring supporting the mass. The second type are position-sensing. These measure the displacement of an accelerated mass connected to a restoring spring.
Both of these methods measure the analog response of the transducer to the acceleration to output a proportional signal. Therefore, the linearity and range of these accelerometers is dependent on the material, architecture and transduction phenomena of the sensor.
These type of accelerometers typically have a sensor element with a mass on the order of a gram. Hybrid techniques are necessary to integrate the transducer with the readout circuitry. These two factors contribute to two problems with this type of accelerometers. They are large in size, by integrated circuit standards. Also, they require expensive tail-end processing, making them expensive by semiconductor industry standards.
These problems have lead to the manufacture of a new type of accelerometers. These typically have masses and springs fabricated from single crystal or polycrystalline silicon. The sensors are on a silicon dioxide membrane. The silicon substrate is etched away with an anisotropic or orientation-dependent etch. The supporting oxide is then freed by wet-etching with hydrofluoric acid.
While this technique is an improvement over the hybrid techniques discussed above, this method produces a single element that is 1 millimeter in size. That measurement does not include the addressing circuitry, which must be put on the side, since the substrate has been removed underneath.
Therefore, it is an object of this invention to provide micro-machined accelerometers.
It is another object of this invention to provide an inexpensive process for manufacturing micro-machined accelerometers including the addressing circuitry.